A 4-year-old boy is brought to the pediatric emergency department by his mother who noticed a large, firm, painless abdominal mass while bathing him. Imaging confirms an intrarenal origin with a well-circumscribed, encapsulated mass. The structure marked **B** in the diagram is suspected. Which of the following is the most characteristic histological feature of this tumor that would be documented in the pathology report?
A. Spindle cell proliferation with bone and cartilage metastases
B. Triphasic histology with blastemal, epithelial, and stromal components
C. Sheets of small round blue cells with Homer-Wright rosettes and N-myc amplification
D. Congenital mesenchymal proliferation presenting before age 1 year
Explanation
Why "Triphasic histology with blastemal, epithelial, and stromal components" is right
The structure marked B is Wilms tumor (nephroblastoma), the most common primary renal malignancy of childhood with peak incidence at age 2–5 years. The classic and most important histological hallmark is triphasic histology comprising: (1) blastemal component (small blue cells representing primitive metanephric blastema), (2) epithelial component (primitive tubules and glomeruloid bodies), and (3) stromal component (spindle cells, sometimes with heterologous skeletal muscle). This triphasic pattern is pathognomonic for Wilms tumor and is essential for diagnosis and prognostication. The presence of all three components, or favorable histology without anaplasia, carries a better prognosis (Nelson Textbook of Pediatrics 22e; COG/SIOP Wilms Tumor Protocols 2023).
Why each distractor is wrong
Sheets of small round blue cells with Homer-Wright rosettes and N-myc amplification: This describes neuroblastoma (marked A), which arises from the adrenal gland and neural crest tissue, not the kidney. Neuroblastoma shows rosette formation and N-myc gene amplification, not the triphasic pattern of Wilms tumor.
Spindle cell proliferation with bone and cartilage metastases: This describes clear cell sarcoma of kidney (marked C), a rare renal malignancy characterized by spindle cell morphology and a predilection for bone metastases. It lacks the triphasic histology of Wilms tumor.
Congenital mesenchymal proliferation presenting before age 1 year: This describes mesoblastic nephroma (marked D), a benign-to-low-grade congenital renal tumor that typically presents in infants under 1 year of age. It does not show triphasic histology and has a much better prognosis than Wilms tumor.
High-YieldNEET PG
Wilms tumor = triphasic histology (blastemal + epithelial + stromal); peak age 2–5 years; most common pediatric renal malignancy; ~90% favorable histology without anaplasia.
Nelson Textbook of Pediatrics 22e; COG/SIOP Wilms Tumor Protocols 2023
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